This invention relates to a method and apparatus for making metal compositions containing degenerate dendrites.
Prior to the present invention, metal compositions have been made containing up to about 65 weight percent degenerate dendrites. Such compositions and their method of preparation are described in U.S. Pat. No. 3,948,650, issued Apr. 6, 1976 to Flemings et al and U.S. Pat. No. 3,954,455, issued May 4, 1976 to Flemings et al. As described by these patents, a metal alloy is heated to form a liquid-solid mixture which is vigorously agitated to convert the dendrites derived from the alloy to degenerate dendrites. The metal alloy is cooled while being vigorously agitated to the point where the viscosity of the high fraction solid material formed by cooling cannot be overcome by increasing shear forces on the high fraction solid material. These compositions can be cast directly or can be further solidified and subsequently reheated to form a thixotropic composition which can be cast directly (thixocasting). Substantial advantages are attained when casting the composition since the mold is not exposed to the heat of fusion of the material solidified prior to casting. Furthermore, the cast material experiences far less shrinkage upon solidification as compared to shrinkage as compared to an article cast from a totally liquid metal composition. However, the process for forming the high fraction solid while continuously agitating it over a significant portion of the cooling cycle causes problems. The degree of agitation required by this process causes undesirable entrapment of gas into the high fraction solid. In addition, the solid degenerate dendrites formed by the present commercial practice of the process contain a large proportion of entrapped liquid formed from eutectic metal compositions. Furthermore, during the cooling cycle, some macrosegregation of solid and liquid occurs. The entrapped gas, entrapped eutectic metal compositions and solid liquid macrosegregation causes the overall metal alloy composition thus formed to be non-uniform with resultant non-uniform physical characteristics such as strength characteristics.
The thixocasting process wherein the high fraction solid material is completely solidified by cooling and then reheated to form a liquid-solid composition is effected in part in order to form more of a microstructure wherein the solid degenerate dendrites are spherodized, resulting in more uniform alloys. However, the process is more costly since the solidified metal in the gates and runners of the forming apparatus must be reheated from the solid state to the liquid state resulting in the loss of the desired degenerate dendrite microstructure.
European Patent Application 96108499.3 (Publication No. EP0745 694A1) discloses a process for forming a liquid-solid metal alloy composition which can be formed by casting. In this process, a melt of the alloy is formed in one or more first vessels. The melt then is transferred to an insulating vessel under cooling conditions wherein crystal nuclei form in the melt. The melt then is further cooled in the insulating vessel under conditions to effect formation of spheroidal solids which form on the nuclei to produce the liquid-solid composition which is then cast. The melt is transferred into the insulating vessel either by moving it over a cooled inclined jig which diverts a melt stream into the insulating vessel or by pouring a plurality of metal alloy melts into the insulating vessel. When utilizing the latter mode of transfer, one of the melts is at a temperature lower than the liquidus temperature of a second melt so that the crystal nuclei form in the second melt. A major problem is that the poured melts entrap gas therein during the transfer and are retained therein due to the presence of solids. This results in a nonuniform final metal alloy composition having defects caused by the entrapped gas. In addition, the cooling rate and degree of agitation are poorly controlled such that the crystal nuclei are limited in number and are not homogeneously dispersed in the liquid melt. This results in degenerate dendrites containing entrapped liquid and and in a formed metal alloy product having nonuniform physical characteristics throughout its volume. Furthermore, a skin is formed on the bottom surface of the solidified product which then must be removed in order to obtain a desired homogeneous final metal alloy product.
European Patent Application No. 95 309498.4 (Publication NO. EPO 719606 A1) also discloses formation of a liquid-solid metal alloy composition by forming an alloy melt in a first vessel and transferring it to an insulating vessel over a cooled inclined surface to form crystal nuclei in the melt. The melt is then cooled to form a liquid-solid alloy composition containing spheroidal solids which can be cast. Since this process relies on a pouring step in the presence of solids, it also has the disadvantage of entrapping gas while forming spheroidal solids nonhomogeneously distributed in the final metal alloy product.
U.S. Pat. Nos. 5,144,998; 5,555,926; 5,901,778 and 5,865,240 also disclose processes for forming a liquid-solid metal alloy composition which effects formation of a metal alloy melt in a first vessel which then is transferred to a second vessel under poorly controlled cooling and agitation conditions to form solid nuclei in the melt. Since the melt is transferred from one vessel to another while partially solidifying the melt, the problems associated with gas entrapment and nonhomogeneous sized spheroidal solids are encountered as set forth above.
Accordingly, it would be desirable to provide a skinless homogeneous liquid-solid metal alloy compositions which are free from entrapped gas and wherein the solid component is free of an eutectic composition. In addition, it would be desirable to form such compositions wherein the primary solids have maximum sphericity which are homogeneously distributed throughout the volume of the metal alloy composition. Such a metal alloy composition would be more easily shaped to provide a product having homogeneous physical characteristics throughout its volume. Furthermore, it would be desirable to provide a process for forming such liquid-solid compositions wherein crystal nuclei are homogeneously dispersed within a liquid melt so that homogeneously sized primary solids can be formed throughout the volume of the liquid-solid metal alloy.
This invention is based on the discovery that a skinless homogeneous liquid-solid metal alloy composition can be formed from a molten metal alloy composition, free of entrapped gas, substantially free of entrapped eutectic in primary solids, and having primary solids which are substantially spherical by controlling conditions of cooling and vigorous agitation of a liquid precursor to the liquid-solid metal alloy. It has been found that rapid cooling and vigorous agitation can be effected for a short time over a narrow temperature range near the liquidus temperature of the molten metal alloy at a controlled cooling rate to form solid particle nuclei. Agitation is then ceased in a batch process or the liquid-solid alloy is removed from the source of agitation in a continuous process while cooling is continued so that the primary solids are formed on the solid particle nuclei while avoiding the formation of a solid dendritic network. The resultant skinless composition comprises homogeneously sized primary spheroidal solid particles substantially free of eutectic metal alloy composition and which is free of entrapped gas. The resultant liquid-solid metal alloy composition that can be formed such as by casting.